When a single-phase induction motor is designed, a magnetic field in the airgap of the motor can be designed to be distributed circularly only at a certain rotation speed. For example, if particular attention is paid to the starting property of the motor, the magnetic field in the airgap of the motor can be designed to be distributed circularly at starting state, but this will result in an elliptical magnetic field during operation; if emphasis is placed upon operation property, the airgap magnetic field at the rated speed can be designed to be circular, but this will result in an elliptical magnetic field during starting of the motor. The elliptical magnetic field will lead to additional loss and noise, which are undesired when the motor is used. That's way the application of single-phase motor is limited, for example in occasions where speed regulation is required.
For multi-phase induction motors including two-phase induction motors, when multi-phase driving voltages are symmetrical, the motor has a circular airgap magnetic field at all rotation speeds so that the motor has characteristics such as high efficiency, silence and high power density. For the multi-phase induction motor, if variable frequency speed regulation is adopted, speed regulation can be perform within a wide range and the efficiency is high. In order to drive a two-phase induction motor or other motors having an even number of phases, 4M number of power semiconductor transistors are required to form a multi-phase bridge, and 2M number of power semiconductor transistors are required to form a multi-phase bridge when there are an odd number of phases. Thus, multiple channels of control signals are required to perform chopping control on these power semiconductor transistors, so as to form multi-phase symmetrical driving currents. All of the above makes the structure of the drive circuit to be complicated, the calculation of the control signal to be difficult, the reliability of the starting system to be poor and the cost of the drive system to be high, which limits the application of motors such as two-phase induction motors.